DX: Add ESLint to backend and wire into CI

[joelpeace48-cell]

Goal\nPrevent backend regressions and enforce consistent code quality.\n\n## Scope\n- Add ESLint config for Node ESM\n- Add script to \n- Update backend CI to run lint (conditionally or always)\n\n## Acceptance criteria\n- runs locally and in CI\n- CI fails on lint errors\n

[STX-Boot]

@STX-Boot has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I would like to work on this, please assign

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[drips-wave]

Congratulations, @STX-Boot! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

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